The present invention relates to a process for inhibiting the corrosion of metal installations in contact with an acid bath. It relates more particularly to a process for inhibiting the corrosion, in contact with aqueous baths containing a mineral acid, of installations made of metals which are less noble than the hydrogen in these baths.
It is known to treat metal installations with acid baths, in particular for the purpose of cleaning them or descaling them. These cleaning and descaling techniques are commonly employed in industry, where they are used in particular for descaling steel boilers and for descaling crystallisation reactors, such as the steel or cast iron columns used for the crystallisation of sodium bicarbonate in the ammonia process for the manufacture of soda, or the nickel evaporators used for the crystallisation of sodium chloride starting from brines of from aqueous solutions of sodium chloride and sodium hydroxide.
The acid baths used in these cleaning and descaling processes must generally contain a corrosion inhibitor, the function of which is to prevent deterioration of the installation without detracting from the cleaning or descaling action of the bath.
The corrosion inhibitors added to the acid baths also serve the purpose of preventing corrosion of the installations used for handling them, in particular the storage vats, the collectors and their accessories, such as gate valves, clack valves and pumps.
In the particular case of steel or cast iron installations, an alkylpyridinium chloride is normally used as the corrosion inhibitor. However, it has been observed, in practice, that despite the presence of the alkylpyridinium chloride in the acid baths, the latter still cause corrosion, and sometimes rapid corrosion, of the installation in certain cases.
A known process for overcoming this disadvantage consists in adding stannous chloride to the bath, in addition to the alkylpyridinium chloride.
However, this known process exhibits the disadvantage of contaminating the bath with tin ions, which are generally considered as dangerous, in particular if the baths are discharged into watercourses.
Another known process for removing a film of oxide from the surface of components made of mild steel consists in treating them with a cleaning paste consisting of a mixture of phosphoric acid, potassium ferrocyanide, urea, zinc phosphate, sawdust, molasses and decylpyridinium chloride (Chemical Abstracts, 1976, Volume 85, No. 97486d).
However, the use of a paste for cleaning industrial installations presents serious difficulties, in particular in the case of installations of large dimensions, or if the paste is used to treat zones of the installation which are not easily accessible.